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<a name="C-Extensions"></a>
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Next: <a href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" accesskey="n" rel="next">C++ Extensions</a>, Previous: <a href="C_002b_002b-Implementation.html#C_002b_002b-Implementation" accesskey="p" rel="prev">C++ Implementation</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Extensions-to-the-C-Language-Family"></a>
<h2 class="chapter">6 Extensions to the C Language Family</h2>
<a name="index-extensions_002c-C-language"></a>
<a name="index-C-language-extensions"></a>

<a name="index-pedantic-2"></a>
<p>GNU C provides several language features not found in ISO standard C.
(The <samp>-pedantic</samp> option directs GCC to print a warning message if
any of these features is used.)  To test for the availability of these
features in conditional compilation, check for a predefined macro
<code>__GNUC__</code>, which is always defined under GCC.
</p>
<p>These extensions are available in C and Objective-C.  Most of them are
also available in C++.  See <a href="C_002b_002b-Extensions.html#C_002b_002b-Extensions">Extensions to the
C++ Language</a>, for extensions that apply <em>only</em> to C++.
</p>
<p>Some features that are in ISO C99 but not C90 or C++ are also, as
extensions, accepted by GCC in C90 mode and in C++.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">&bull; <a href="Statement-Exprs.html#Statement-Exprs" accesskey="1">Statement Exprs</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Putting statements and declarations inside expressions.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Local-Labels.html#Local-Labels" accesskey="2">Local Labels</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Labels local to a block.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Labels-as-Values.html#Labels-as-Values" accesskey="3">Labels as Values</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Getting pointers to labels, and computed gotos.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Nested-Functions.html#Nested-Functions" accesskey="4">Nested Functions</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Nested function in GNU C.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Nonlocal-Gotos.html#Nonlocal-Gotos" accesskey="5">Nonlocal Gotos</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Nonlocal gotos.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Constructing-Calls.html#Constructing-Calls" accesskey="6">Constructing Calls</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Dispatching a call to another function.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Typeof.html#Typeof" accesskey="7">Typeof</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top"><code>typeof</code>: referring to the type of an expression.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Conditionals.html#Conditionals" accesskey="8">Conditionals</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Omitting the middle operand of a &lsquo;<samp>?:</samp>&rsquo; expression.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="_005f_005fint128.html#g_t_005f_005fint128" accesskey="9">__int128</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">128-bit integers&mdash;<code>__int128</code>.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Long-Long.html#Long-Long">Long Long</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Double-word integers&mdash;<code>long long int</code>.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Complex.html#Complex">Complex</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Data types for complex numbers.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Floating-Types.html#Floating-Types">Floating Types</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Additional Floating Types.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Half_002dPrecision.html#Half_002dPrecision">Half-Precision</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Half-Precision Floating Point.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Decimal-Float.html#Decimal-Float">Decimal Float</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Decimal Floating Types.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Hex-Floats.html#Hex-Floats">Hex Floats</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Hexadecimal floating-point constants.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Fixed_002dPoint.html#Fixed_002dPoint">Fixed-Point</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Fixed-Point Types.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Named-Address-Spaces.html#Named-Address-Spaces">Named Address Spaces</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Named address spaces.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Zero-Length.html#Zero-Length">Zero Length</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Zero-length arrays.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Empty-Structures.html#Empty-Structures">Empty Structures</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Structures with no members.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Variable-Length.html#Variable-Length">Variable Length</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Arrays whose length is computed at run time.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Variadic-Macros.html#Variadic-Macros">Variadic Macros</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Macros with a variable number of arguments.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Escaped-Newlines.html#Escaped-Newlines">Escaped Newlines</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Slightly looser rules for escaped newlines.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Subscripting.html#Subscripting">Subscripting</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Any array can be subscripted, even if not an lvalue.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Pointer-Arith.html#Pointer-Arith">Pointer Arith</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Arithmetic on <code>void</code>-pointers and function pointers.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Variadic-Pointer-Args.html#Variadic-Pointer-Args">Variadic Pointer Args</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Pointer arguments to variadic functions.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Pointers-to-Arrays.html#Pointers-to-Arrays">Pointers to Arrays</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Pointers to arrays with qualifiers work as expected.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Initializers.html#Initializers">Initializers</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Non-constant initializers.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Compound-Literals.html#Compound-Literals">Compound Literals</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Compound literals give structures, unions
                        or arrays as values.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Designated-Inits.html#Designated-Inits">Designated Inits</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Labeling elements of initializers.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Case-Ranges.html#Case-Ranges">Case Ranges</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">&lsquo;case 1 ... 9&rsquo; and such.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Cast-to-Union.html#Cast-to-Union">Cast to Union</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Casting to union type from any member of the union.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Mixed-Declarations.html#Mixed-Declarations">Mixed Declarations</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Mixing declarations and code.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Declaring that functions have no side effects,
                        or that they can never return.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Specifying attributes of variables.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Type-Attributes.html#Type-Attributes">Type Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Specifying attributes of types.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Label-Attributes.html#Label-Attributes">Label Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Specifying attributes on labels.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Enumerator-Attributes.html#Enumerator-Attributes">Enumerator Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Specifying attributes on enumerators.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Statement-Attributes.html#Statement-Attributes">Statement Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Specifying attributes on statements.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Attribute-Syntax.html#Attribute-Syntax">Attribute Syntax</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Formal syntax for attributes.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Function-Prototypes.html#Function-Prototypes">Function Prototypes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Prototype declarations and old-style definitions.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="C_002b_002b-Comments.html#C_002b_002b-Comments">C++ Comments</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">C++ comments are recognized.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Dollar-Signs.html#Dollar-Signs">Dollar Signs</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Dollar sign is allowed in identifiers.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Character-Escapes.html#Character-Escapes">Character Escapes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">&lsquo;<samp>\e</samp>&rsquo; stands for the character <tt class="key">ESC</tt>.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Alignment.html#Alignment">Alignment</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Determining the alignment of a function, type or variable.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Inline.html#Inline">Inline</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Defining inline functions (as fast as macros).
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Volatiles.html#Volatiles">Volatiles</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">What constitutes an access to a volatile object.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C">Using Assembly Language with C</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Instructions and extensions for interfacing C with assembler.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Alternate-Keywords.html#Alternate-Keywords">Alternate Keywords</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top"><code>__const__</code>, <code>__asm__</code>, etc., for header files.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Incomplete-Enums.html#Incomplete-Enums">Incomplete Enums</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top"><code>enum foo;</code>, with details to follow.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Function-Names.html#Function-Names">Function Names</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Printable strings which are the name of the current
                        function.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Return-Address.html#Return-Address">Return Address</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Getting the return or frame address of a function.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Vector-Extensions.html#Vector-Extensions">Vector Extensions</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Using vector instructions through built-in functions.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Offsetof.html#Offsetof">Offsetof</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Special syntax for implementing <code>offsetof</code>.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="_005f_005fsync-Builtins.html#g_t_005f_005fsync-Builtins">__sync Builtins</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Legacy built-in functions for atomic memory access.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="_005f_005fatomic-Builtins.html#g_t_005f_005fatomic-Builtins">__atomic Builtins</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Atomic built-in functions with memory model.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Integer-Overflow-Builtins.html#Integer-Overflow-Builtins">Integer Overflow Builtins</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Built-in functions to perform arithmetics and
                        arithmetic overflow checking.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="x86-specific-memory-model-extensions-for-transactional-memory.html#x86-specific-memory-model-extensions-for-transactional-memory">x86 specific memory model extensions for transactional memory</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">x86 memory models.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Object-Size-Checking.html#Object-Size-Checking">Object Size Checking</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Built-in functions for limited buffer overflow
                        checking.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Other-Builtins.html#Other-Builtins">Other Builtins</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Other built-in functions.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Target-Builtins.html#Target-Builtins">Target Builtins</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Built-in functions specific to particular targets.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Target-Format-Checks.html#Target-Format-Checks">Target Format Checks</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Format checks specific to particular targets.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Pragmas.html#Pragmas">Pragmas</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Pragmas accepted by GCC.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Unnamed-Fields.html#Unnamed-Fields">Unnamed Fields</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Unnamed struct/union fields within structs/unions.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Thread_002dLocal.html#Thread_002dLocal">Thread-Local</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Per-thread variables.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Binary-constants.html#Binary-constants">Binary constants</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Binary constants using the &lsquo;<samp>0b</samp>&rsquo; prefix.
</td></tr>
</table>

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